No. 5 Maria Sharapova: Serena Williams

Maria Sharapova is enjoying the best year of any player on the WTA Tour. Her win at the French Open propelled her back to the top five. It also put her ahead of all players in points earned.

Yet, you wonder how many accolades Sharapova would trade for one victory over Serena Williams? Sharapova has a 2-16 record against Williams and has lost 15 in a row. Her last victory over Williams was in 2004, the year she won her only Wimbledon title.

If they meet in the quarterfinals. the match may be built as the 10th anniversary of Sharapova's upset win over Williams. That type of hype could bolster Sharapova or irk Williams. Either way, getting past this obstacle won't be easy for Sharapova.

No. 5 Stanislas Wawrinka: Inconsistency

Stanislas Wawrinka has reached the third round at Wimbledon for the first time since 2009.

He had been bounced in the first round in three of the past four years. That's incredible considering he has the big serve and powerful ground strokes usually associated with great grass-court players.

But that's Wawrinka, unpredictably inconsistent. One tournament he looks like the Grand Slam champion he is. The next, he crashes in the first round. His horrible record at Wimbledon was a factor in being seeded No. 5, despite being ranked No. 3.

No. 4 Agnieszka Radwanska: Big Hitters

Associated PressAgnieszka Radwanska gets low to hit a backhand in her first-round match.

Agnieszka Radwanska is one of the most graceful players on tour. Even her service toss is elegant. She is also sneaky fast. She's lucky grass rewards the quick. Unfortunately, grass also favors powerful big hitters.

That's where it gets tricky for Radwanska.

She defends well, but has no distinguishable weapons to hurt the heavy hitters. Her consistency, shot selection and superb movement keeps her in the top five. But when under attack by top players, she eventually withers.

Radwanska is quite comfortable on grass. But her two most recent, and perhaps most heart-breaking losses—last year's semifinal loss to Sabine Lisicki and a loss in the finals to Serena Williams—have come at the hands of big hitters with massive serves.

No. 4 Roger Federer: Fatigue

One of the greatest grass-court players in the history of the game, Federer is always a contender to win at Wimbledon.

The seven-time Wimbledon champion plays with such precision on grass that when he takes to the court you expect greatness. Most of the time he delivers. Federer fired 25 aces and 42 winners in his second-round match win over Gilles Muller.

Federer still has those beautiful strokes. His one-handed backhand remains lethal. But in longer matches and lengthy rallies, Federer sometimes shows signs of fatigue.

Fatigue is the biggest obstacle facing Federer. The father of two sets of twins, including infant boys, Federer maintains that he is in great shape these days. He will need to be if he gets into a five-setter with Wawrinka, Rafael Nadal, Novak Djokovic or Andy Murray.

No. 3 Simona Halep: Inexperience

Al Bello/Getty ImagesSimona Halep stares down the ball as she readies her backhand.

Simona Halep's swift rise to No. 3 has had a few bumps along the way. But if there's one speed bump that may slow her roll, it's inexperience.

Halep, 22, is the youngest player in the top five. With each big match she gains confidence. Apparently a student of the game, Halep will need to be a quick studier to advance to the head of the class.

Notorious for mental gaffs, Li once served a ball into the stands at the Australian Open. The serve flew so far away from the court that it is considered one of the worst in the history of tennis. Earlier this year, in Miami, she led Serena Williams 5-2 in the first set and lost that match 7-5, 6-1.

No. 2 Rafael Nadal: His Draw

Al Bello/Getty ImagesRafael Nadal gets real low to make a play on the ball during his first-round match at Wimbledon, 2014.

By surviving another tough match against Lukas Rosol, Nadal cleared an early hurdle. However, bigger obstacles remain in his half of the draw.

Gigantic servers like John Isner and Milos Raonic have just the type of game that could spell trouble for Nadal. Even if he gets past the big guys, Federer or Wawrinka might be waiting. It's a brutal draw.

No. 1 Novak Djokovic: His Wrist

Steve Bardens/Getty ImagesNovak Djokovic gets up from a fall during his second-round match at Wimbledon.

Djokovic's nagging wrist injury made news again just prior to Wimbledon. Djokovic aggravated the injury in Monte Carlo in April. He withdrew from the Madrid Masters and told reporters that he feared it would take "some time to recover."

However, he was able to return sooner than expected. He won in Rome and reached the finals in Roland Garros. So far there's been no evidence that his wrist is a factor. But as the tournament goes on, it could hamper his efforts to win his second Wimbledon title.